You probably messed up by not having the slashes correct. You should use fullfile(), like this:
folder = 'E:\matlab';
if ~isfolder(folder)
% Folder does not exist. Ask the user to browse to an existing one.
uiwait(msgbox('Please select the output folder'))
folder = uigetdir(pwd);
if folder == 0
% User clicked cancel.
return;
end
end
for k = 1 : whatever
% Some code to change the current figure...
% Now save the current figure.
baseFileName = sprintf('%03d.png', k);
fullFileName = fullfile(folder, baseFileName);
saveas(gcf, fullFileName);
end
